Chemical plating



United States Patent Ofiice 3,373,054 Patented Mar. 12, 1968 3,373,054 'CHEMICAL PLATING Konrad Lang, Cologne-Stammheim, and Heinz-Giinter Klein, Cologne-Deutz, Germany, assignors to Farhenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a German corporation No Drawing. Filed June 18, '1964, Ser. No. 376,207 Claims priority, application Germany, July 22, 1963, F 40,295 9 Claims. (Cl. 117130) ABSTRACT OF THE DISCLOSURE An improved process for chemical plating with alkaline aqueous plating baths containing a nickel and/or cobalt salt, a primary complexing agent such as ethylene diamine and a boron hydride compound is disclosed. The mprovement is directed to the regeneration of such plat mg baths, the same to be carried out in the plating bath itself and without interrupting the process. The improvement in accordance with the invention lies in the teaching to add to the bath during the plating process an aqueous solution containing a secondary complex formmg agent, the same being characterized in that it is volatile at the temperature at which the plating operation is being carried out; the primary complexing agent not being volatile at such temperature and further the secondary complexing agent being characterized in that it forms less stable complexes with the metal salt than does the primary complexing agent, together with a metal salt, i.e., nickel and/r cobalt salt.

The present invention relates to a process for the regeneration of plating baths, containing boron hydride compounds as reducing agents.

It is an object of the present invention to provide a new and highly economic process for the regeneration of chemical plating baths containing boron hydrides as reducing agents.

It is another object of the present invention to provide a process for the regeneration of plating baths which can be performed in the baths itself.

It is still another object of the present invention to provide a process for the regeneration of plating baths which can be carried out without interrupting the plating process.

Still further objects of the present invention will become apparent as the following description proceeds. In chemical nickel or cobalt plating with the aid of boron hydride compounds, especially the easily accessible sodium borohydride as reducing agent plating baths are used which contain, in addition to the reducing agent a nickel or cobalt salt, e.g. chloride, a complex forming agent, e.g. ethylene diamine, and an alkali metal hydroxide, e.g. sodium hydroxide as well as stabilizers etc.

During the deposition of metal, the bath becomes enriched in alkali metal borate and alali metal salt (chloride) while the content of nickel salt and reducing agent falls. To maintain optimum conditions for plating the bath must constantly be replenished with the reducing agent and with the metal to be deposited, added in the form of its complex salt, and at least a part of the byproducts must be removed. According to Belgian patent specification 618,098, this may suitably be done by continuously removing from the plating bath a relatively small quantity of the bath liquid, removing borate and chloride from this by treating it with a solution of an alkaline earth metal salt and then adding nickel salt, alkali metal hydroxide and if necessary complex-forming agents; the regenerated solution can then be returned to the plating bath. This method is particularly advantageous for plants of medium to large size which operate continuously. In smaller units, on the other hand, which are operated intermittently, there is frequently a tendency to waive the regeneration plant and to discard the plating baths after they. have been used for some time. This method, however, is economical only if the bath can be utilized to its utmost.

The invention relates to a process for chemical plating with alkaline aqueous plating baths containing nickel and/or cobalt salts, primary complex-forming agents as hereinafter defined and boron hydride compounds, the process which comprises adding to said baths during the plating process an aqueous solution with a secondary complex-forming agent as hereinafter defined, said agent having a lower complex stability and a higher volatility than the primary complex-forming agents and a nickel and/or cobalt salt at the rate at which these metal salts are reduced and deposited.

It has been found that a bath based on borohydride compounds used for chemical plating, for example a nickel plating bath, consisting of a solution in'aqueous sodium hydroxide of ethylene diamine, sodium borohydride and nickel chloride, optionally with the addition of stabilizers, e.g. lead salts, Which loses nickel salt in the course of the plating process but still contains about 1 mol per litre alkali metal hydroxide and reducing agents can be readjusted to the starting concentration by the addition of an aqueous solution of nickel chloride together with a volatile, weaker complcx-forming agent and can be operated in the same way as a freshly prepared bath. Suitable complex-forming agents for the process of inventi0n-they shall be denoted as secondary complex-forming agents-are all amines or their aqueous solutions which are volatile at the temperature at which the plating bath operates, 40100 C. Accordingly e.g. ammonia, methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, n-propylamine, iso-propyl amine, n-butylamine, iso-butylamine etc. can be used.

The nickel salt/amine solution may be added to the bath in portions, for example after each batch has been plated, or the solution may be added continuously. The secondary complex-forming agents are used in such amounts that the corresponding nickel hexamine or cobalt tetramine complexes are formed, i.e. in molar ratios of Ni-amine of about 1:6 and of Co-amine of about 1:4. Minor deviations of these ratios are not harmful. Larger additions of amine are not useful for reason of economy. The alkali metal hydroxide which is also removed from the bath during the plating process is supplemented by the addition of a solution of alkali metal hydroxide, preferably simultaneously with the reducing agent.

As metal salts there can be used water-soluble nickel or cobalt salts such as nickel chloride, nickel sulfate, cobalt chloride or cobalt sulfate etc.

As reducing agents there can be used borohydride compounds such as alkali metal borohydrides, preferably sodium borohydride, borazanes'or 'borazoles, substituted with hydrogen on the !boron atom.

Fresh plating baths are prepared with complex-forming agents-primary complex-forming agents-which form more stable complexes with nickel and cobalt salts than the secondary complex-forming agents and which agents are not volatile under the temperature conditions of above 40 to C. used for the plating process. As so called primary complex-forming agents e.g. ethylene diamine, triethylene tetramine, diethylene triamine', tetraethylene pentamine etc. can be used.

No precipitation of basic nickel salt or nickel hydroxide occurs in this method during the addition of the nickel salt replacement solution in Spite of the high concentration of alkali metal hydroxide in the plating solution. There is therefore also no accumulation of complex-forming agents in the bath which could disturb the plating process. Spontaneous decomposition of the bath is also out of question.

Thus, for example, in a bath for chemical nickel plating (volume, 1 litre) containing nickel chloride, ethylene diamine and sodium hydroxide, the nickel salt concentration was lowered in the course of the plating process from an initial 7.5 g. Ni/l. to about 0.7 g. Ni/l. The nickel content was raised to the original value by the addition of a solution of 28 g. nickel chloride hexahydrate in 100' ml. of an approximately 24% ammonia solution. In addition 9.5 g. sodium hydroxide were added. The apparent reduction yield, i.e. the yield corresponding'to the equation was 35% in this replenished bath as compared with 37% in the original bath. A similar yield was obtained even after a second and third renewal of the bath. Only after the fourth replenishment did the reduction yield fall to about 26%. In case ethylene diamine was used instead of ammonia as complex-forming agent in the supplementing solution, the speed of deposition and hence also the reduction yield fell very sharply owing to the accumulation of complex-forming agents, and in some cases the plating process even came to a standstill. If nickel salt solutions free from complex-forming agents were added, the basic salt or nickel hydroxide was precipitated at the point of inflow, which led to the decomposition of the bath in the presence of alkali metal borohydride.

Thus in a discontinuously operated plating bath it is possible, by the method described, to put through a quantity of nickel corresponding to 4 to 5 times the original concentration of nickel without any regenera tion and without the speed of plating and the reduction yield being significantly impaired.

The process of the invention is, however, not limited to discontinuously operated baths. In fact, when continuously operating plating baths are used, it is possible to put through the bath a quantity of nickel equal to ten times the original concentration before any regeneration with alkaline earth metal salts according to Belgian patent specification 618,098 becomes necessary.

Chemical plating is simplified and rendered more economical by. the process of the invention because in spite of constant addition of a complex nizkel salt so'ution, there is no accumulation of complex forming agents in the plating baths which would inhibit the plating process and continuous removal of bath fluid is unnecessary and regeneration of the plating bath need be carried out less frequently and moreover, the consumption of chemicals is reduced.

Example 1 Six chemical plating baths a, b, c, d, e and f of a volume of l l. and having the composilions: 3O g./l. nickel chloride, 40 g./l. sodium hydroxide, 60 g./l. ethylene diamine (approximately 98%), 3 g./l. sodium fluoride and 0.5 g./l. sodium borohydride are heated to 90 C. Four degreased and pickled iron plates each havmg a surface area of x 10 cm. are suspended in each bath. .The lead chloride solution containing 2 g. PbCl per litre is added dropwise at the rate of 40 ml./h. Portions of 0.5 g. sodium borohydride dissoved in approximately 50 ml. cooled bath solution are added to the plating bath at half-hourly intervals. Approximately 6 g. nickel have been deposited on the iron plates from each plating bath after 3 hours. The concentration of nickel has thus dropped to 20% of the original value in each bath.

Six portions of 24 g. nickel chloride (NiCl -6H O) are now dissolved each in 50 ml. water and the nickel salt solutions are introduced, with cooling, into the various baths as follows: For bath a into 100 ml. methylamine, for bath [2 into 200 ml. dimethylamine, for bath 0 into 200 ml. trimethylamine, for bath d into 200 ml. 40% ethyla-mine, for bath e into 200 ml. 65% isopropylamine and for bath 1 into 200 ml. 96% n-butylamine. A clear solution is obtained only when the nickel salt solution is poured into methylamine. By adding this nickel-amine complex salt solution or suspension to the plating baths which have been cooled to room temperature, the nickel salt in the baths is readjusted to the original concentrations. The bath liquors obtained need not be filtered.

After heating to about 93 C., 0.5 g. sodium borohydride dissolved in about ml. bath liquor is added to all the baths and four iron plates are again suspended in each bath and plated for 3 hours as in the original bath. In all the six baths, the quantity of nickel deposited on the iron plates during this time is equal to the quantity deposited from the freshly prepared plating baths. All the plated baths had a smooth and bright Ni-B coating.

ExampleZ 1 l. of a chemical plating bath consisting of 33 g./l. nickel sulfate, 87 g./l. triethylene tetramine and 40 g./l. sodium hydroxide is heated to 95 C. and two brass plates and two copper plates each having a surface of 10 cm. x10 cm. are suspended in the bath and 0.6 g. NaBI-L, dissolved in about 50 ml. bath solution is added. 20 ml. lead chloride solution containing 2 g. PbCl per 1. and 0.6 g. sodium borohydride dissolved in 50 ml. bath solution are added at 20 minute intervals to the plating bath. Plating is continued for a period of 3 hours. 4.75 g. Ni-B alloy are deposited on the four plates at the end of this time. 22.8 g. nickel sulfate (NiSO -7H O') are then dissolved in 50 cc. water and poured into 100 cc. ammonia solution (approximately 25% NH The nickel amine sulfate is then added to the plating bath, the bath is heated to the plating temperature and nickel plating is continued as described above. In the course of 3 hours, 4.90 g. Ni-B alloy are deposited on the plates. The coatings are smooth and bright.

Example 3 0.6 g. sodium borohydride dissolved in 50 ml. bath solution is added to 1 litre of a plating bath of the composition 30 g./l. nickel chloride, 40 g./l. sodium hydroxide, g./l. ethylene diamine (98%) and 10 g./l. sodium fluoride, which is heated to 90 C., and two iron and two brass plates each having a surface of 10 cm. x 10 cm. are suspended in the bath. 20 ml. of a lead acetate solution containing 2 g. Pb(CH COO) per litre is continuously added dropwise over a period of 30 minutes. 0.6 g. NaBH dissolved in approximately 50 ml. bath solution is added to the bath at half-hourly intervals. The plating time is 3 hours. At the end of this time, a total of 5.99 g. Ni-B alloy has been deposited on the plates. The plating bath is replenished by the. addition of 1300 cc. of a solution containing 24 g. NiCl -6H O and cc. conc. ammonium solution (approximately 25% NHg). The alkali content of the plating bath is readjusted with 40 cc. of sodium hydroxide solution containing 200 g. NaOH per 1. Nickel plating is then continued for 3 hours. The plated plates are then weighed back and the bath, which is now poor in nickel and alkali, is again replenished as described above and plating is again continued. This process is repeated altogether five times. The deposition of nickel over a plating time of 3 hours is in each case as follows.

Quantity of Ni-B Plating time in hours: alloy deposited g.

3 5.9879 1) Replenishment of bath, 3-6 +5.8720 (2) Replenishment of bath, 69 +5.9683 (3) Replenishment of bath, 9-12 +5.9351 (4) Replenishment of bath, 12-15 +4.8972 (5) Replenishment of bath, 15-18 +4.3296

No decomposition of the bath occurs during plating. The plating bath need not be filtered. The coatings obtained on the plates are smooth and bright.

Example 4 A chemical plating bath (volume 1 l.) of the composition 30 g./l. nickel chloride, 40 g./l. sodium hydroxide, 60 g./l. ethylene diamine, g./l. sodium fluoride and 0.6 g./l. sodium borohydride is operated at 90 C. Four metal plates each having a surface of 10 cm. x 10 cm. are suspended in the bath.

The following are added continuously during plating: 40 ml./h. of an aqueous solution of- G./l. Sodium borohydride 52.5 Sodium hydroxide 117.5 Lead acetate 4.0

80 ml./ h. of a solution of- Nickel chloride 157.0 Ammonia conc. (approximately 25%) 500.0

Example 5 It is desired to coat chemically with an Ni-B alloy the inner walls of two steel pressure flasks provided with closure caps, as Well as the thread of the valves and the outer surfaces on which the caps sit.

For this purpose a closely fitting rubber tube about cm. long is pulled as a support over the thread for the closure cap. The surface to be plated is first treated for minutes at 90 C. with an alkaline degreasing agent, pickled for about 10 minutes with half concentrated hydrochloric acid and again rinsed. The steel flask is then placed in the hot Water bath and filled with 11 l. of plating solution of the following composition, which is heated to about 90 C.:

G./l. Nickel chloride Sodium hydroxide Ethylene diamine (98%) 60 Sodium fluoride 3 Lead acetate 0.3 Sodium borohydride 0.7

The plating temperature is 90 to 91 C. 60 g. NaBH in which approximately 200 cc. plating liquid are dissolved and 100 cc. lead acetate solution containing 3 g. lead acetate per litre are introduced in each case after 20 minutes through a glass funnel whose outlet level is adjustable. The plating time is 2.5 hours.

When plating is finished, the plating liquid is run oif. The steel flask is rinsed several times and then dried. Analysis of the depleted bath liquor indicated 1.2 g./l.

NiCl -6H O. The used plating bath liquor is readjusted to the original concentration of 30 g. NiCl -6H O per litre by the addition of 317 g. nickel chloride dissolved in 1 litre conc. ammonia solution. The solution thus obtained is heated in the course of about half hour to C. 6 g. NaBHI are added and the solution is then filled into another steel flask which has been carefully degreased, pickled and rinsed and preheated in the water bath.

Plating is carried out as described above. After a nickel plating time of 2.5 hours, no more nickel can be detected in the plating solution.

Here again the Ni-B coating deposited on the inner wall of the flask is smooth, even and bright. The thickness of the layer is approximately 25 We claim:

1. In a process for chemical plating with an alkaline plating :bath containing a metal salt selected from the group consisting of nickel and cobalt salts, a primary complex forming agent selected from the group consisting of poly lower alkylene polyarnines and a boronhydride compound, the improvement which comprises adding to said bath during the plating process an aqueous solution containing a secondary complex forming agent selected from the group consisting of ammonia, mono-, diand trialkyl amines, said secondary complexing agent being volatile at the temperature at which said plating process is being carried out, said primary complexing agent not being volatile at said temperature, said primary complexing agent forming more stable complexes with said metal salt than said secondary complexing agent and a metal salt selected from the group consisting of nickel and cobalt salts at a rate at which said metal salt is reduced and deposited.

2. Improvement according to claim 1 wherein said secondary complexing agent is selected from the group consisting of amines having from 1 to 4 carbon atoms and a boiling point of less than C.

3. Improvement according to claim 1 wherein said boronhydride compound is sodium borohydride.

4. Improvement according to claim 1 wherein said secondary complexing agent is a member selected from the group consisting of methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, N-propylamine, isopropylamine, N-butylamine and isobutylamine.

5. Improvement according to claim 1 wherein said process is effected at a temperature of from 40 to 100 C.

6. Improvement according to claim 1 wherein said salt is a member selected from the group consisting of nickel chloride, nickel sulfate, cobalt chloride and cobalt sulfate.

7. Improvement according to claim 1 wherein said primary complexing agent is ethylene diamine and said secondary complexing agent is methylamine.

8. Improvement according to claim 1 wherein said primary complexing agent is triethylene tetraamine and said secondary complexing agent is ammonia.

9. Improvement according to claim 1 wherein said primary complexing agent is ethylenediamine and said secondary complexing agent is ammonia.

References Cited UNITED STATES PATENTS 2,766,138 10/1956 Talmey l17--130 X 2,819,187 1/1958 Gutzeit et al 1061 X 2,916,401 12/1959 'Puls et al. ll7-130 3,096,182 7/1963 'Berzins 1l7-l30 X 3,150,994 9/1964 Hoke 117-130 ALFRED L. LEAVITT, Primary Examiner. C. K. WEIFFENBACH, Assistant Examiner. 

